Contact devices



*Nov. 22, 1966 K. J. AVERSTEN 3,287,037

CONTACT DEVICES Filed Dec. 17, 1964 INV EN TOR KARL J. AVERSTEN BY J5me.Q Day/93 ATTORNEYS United States Patent 3,287,037 CONTACT DEVICES KarlJoel Aversten, Lidingo, Sweden, assignor to AGA Aktiebolag, Lidingo,Sweden, a corporation of Sweden Filed Dec. 17, 1964, Ser. No. 418,967Claims priority, appliczastion Sweden, Jan. 10, 1964, 3 Claims. (Cl.28720.2)

The present invention relates to electrical contact devices. Moreparticularly, the present invention relates to contact devices having atleast one contact shoe which is soldered to a metallic surface by meansof an aperture having inserted therein a metal stud provided with acylinder of solder metal at its end adjacent the surface to which thecontact is to be secured. This type of contact is secured to a metalsurface by creating an electric are between the solder metal of the studand the metallic surface. The are is utilized not only for melting thesolder metal in the aperture of the cable shoe but also for heating theportions of the shoe nearest to the arc as well as the metallic surface.In this manner, the solder metal provides a solid joint between theseparts and the metal stud whenthe latter has been forced into the soldermetal in the aperture and the solder metal has been allowed to solidify.A contact device of this type and the attachment thereof is described inthe US. patent specification 2,711,915.

In order to provide an effective soldered joint, the soldering surfacesto which the solder metal should adhere after having solidified musthave a temperature substantially higher (for instance 200 C. higher)than the melting temperature of the solder metal. It is thereforecustomary to preheat the soldering surfaces with the aid of a separateheat source before applying the molten solder metal to the surfaces.However, such preheating should not take place in connection with theattachment of the contact devices but all heating should be obtained inconnection with sustaining the electric arc. The electric arc must bemaintained during the time required for all of the solder metal on thestud to melt off, because the quality of the solder connection isimpaired if the metal stud itself melts as may occur if the meltinginterval is too long. The quality of the joint may also be poor ifunmelted solder remains in the joint as will occur if the meltinginterval is too short. However, the melting of all of the solder metalmust also be consistent with the application of such an amount of heatas to cause the soldering surfaces to reach the required temperature.Therefore, up to now, the required quality of contact device wasobtained only after a series of tests, but without there being anyprecise knowledge as to the limits of dimensions of the contact devicesof this type. This is probably the reason why some soldered connectionsin practical applications have proved to have unsatisfactory strengthand even occasional failures have occurred.

It is an object of the present invention to provide a contact devicewhich is free from the foregoing disadvantages. This and other objectsare achieved according to the present invention by proportioning thecontact shoe relative to the cross-section, height and melting point ofthe solder metal provided on the metal stud. More precisely, the contactshoe should be proportioned to have a volume according to a formula tobe stated below.

The invention will be explained with reference to an embodiment shown onthe attached drawing wherein like numerals refer to like parts andwherein:

FIG. 1 is a vertical longitudinal section through a portion of a contactdevice.

FIG. 2 is a plan view of the device.

FIG. 3 is a lateral view of a solder metal stud.

lCC

FIG. 4 is a similar section to FIG. 1 but with the stud in its positionat the start of the melting off process.

. FIG. 5 is a similar section as FIG. 4 when the contact device has beenattached.

The drawings show a cable 1 and a contact shoe 2 of metal, e.g. copper,which is to be soldered to a metallic surface such as a rail 3, as seenin FIGS. 4 and 5. The cable shoe 2 has an outer radius R and an apertureof radius r and a thickness I, so that the volume W thereof is 1r(R r(t) The solder metal stud shown in FIG. 3 comprises a metal stud 4 of,e.g., brass and a cylinder of soldering composition 5 which may comprisea silver solder having a melting temperature of 610 C. Thecross-sectional area of the composition 5 is designated A and its axialdimension h. The radius of the composition 5 is about 0.5 to 1 mm.shorter than the radius r. The following is an example of a suitablecomposition of the silver solder:

Cu 19%, Zn 13 to 19%, Ag 44 to 46%, Cd 18 to 22%.

FIG. 4 shows the solder metal stud 4, 5 being held above the aperture ofthe cable shoe. Upon application of the current, an arc is struckbetween the solder metal 5 at the forward end of the stud and the rail 3and heat is transferred from the arc and from melted solder metal to therail 3 and the cable shoe 2. When all the solder metal 5 has melted off,the stud 4 is forced into the aperture of the cable shoe and into themolten solder metal. Simultaneously, solder metal has flown outwardlyunder the cable shoe. The whole planar underside of the shoe andcorresponding planar portions of the rail, therefore, form solderingsurfaces which have to reach a temperature substantially higher than themelting temperature of the solder metal, i.e. 610 C., but should notreach the melting temperature of the cable shoe, which, in the case ofcopper, is 1085 C. To meet these requirements, it is necessary tomaintain the heat dissipation from the soldering surfaces withindetermined limits. The dissipation of the rail 3 cannot be controlled toany noticeable extent, but this does not apply to the cable shoe, whichshould, therefore, according to the invention be proportioned to have aminimum volume according to the following formula:

ow min- EgT) and a maximum volume according to the formula:

mai ram wherein, in each formula:

max. ge l/ Upon selection of a solder metal stud and a cable shoe of acertain material it is there-fore a simple matter to insert thecorresponding values in the above formulas and to ascertain the limitsof the volume of the cable shoe. There remains a determination of thethickness of the cable shoe, but this is a simple matter, since itcannot very will be thinner than /3 of the diameter thereof in view ofthe required strength and it is not practical to make it thicker than Aof the diameter of the shoe in view of the length of the aperture of theshoe and the requirement for the arc not to reach the Walls of theaperture but only the rail at the bottom thereof.

In view of these formulas, it is therefore easy to proportion the cableshoe so as to obtain a solid and dependable 4. point of 610 C., adiameter of a 2.0 cm. and a height of 0.5 cm., a copper shoe having amelting point of about 1085 C. according to the present invention has amaximum volume of about 4.42 cm. and a minimum volume solder connection,since the cable shoe is given a suitable 5 r of 1.73 CD13. Rememberingthat 2r is slightly larger dimension of soldering surfaces in view ofthe mechanical than the diameter of the solder and assuming that thestrength requirements to 'be met and reaches a suflicient thickness t ofthe shoe is A of the radius R (correspondworking temperature for bondingwith the solder metal ing to A; of the shoe diameter) then the shoehas'a without the temperature being so high as to cause meltradium R ofabout 1.6 cm. and a thickness of about 0.4 ing of the cable shoe.Furthermore, it is ensured that cm. to correspond with the minimumvolume and a radius the solder surface of the rail reaches a suflicienttern- R of about 2.0 cm. and corresponding thickness for maxiperaturefor the bonding of the solder metal owing to mum volume.

the diffusion taking place. I claim:

The circumference of the cable shoe 2 has been shown 1. A contact devicefor soldering to a metallic surface in the top view to be circular andconcentric with the comprising a Contact Shoe having all apertureProvided aperture, but it is obvious that it may also be of square in nWhich iS inserted a metal Stud having a or hexagonal shape etc, Thecable 1 joins thg cable h cylinder of solder metal at its forward end,said contact at its circumference and therefore dissipates some of theShoe being proportioned to have a minimum Volume heat therefrom but thisinfluence isso small that it is not according to the following formula:necessary to take it into account when applying the formulas. Thedrawing shows only one cable shoe of a r/s cable, but it is obvious thata complete rail contact device W,,,.,, (is) (A) comprises a cable havinga cable shoe at each end thereof, (Kl 100) both of which areproportioned according to the above formulas. Even if there are twoapertures and two solderand a maximum Volume according to the followinging studs utilized for each cable shoe, the formulas may formula: servefor the proportioning of the cable shoes.

It has been found that the preferable value of k is (1 fill/3) dependenton the composition of the solder used, More W =(k)( A) log,,(K /100)particularly, it appears that the preferable value of k is ge a functionof the product of the specific heat and the specific gravity for thesolder in question. However, since wherein, in each formula: a solder iscomposed of several metals, this product has to be taken for eachcomponent separately and a W=th v lume of the cable'shoe in mfi,

welghted meal} Value 15 to be Used, Which s Calculated A=thecross-sectional area of the solder metal stud in. with the proportionsof the various components as weightcm. ing factors. If the Weightedmeans value of the products h=th height of th cylinder of tal i 15deslgneted Specific heat pe gravlty g and K=the melting temperature ofthe solder metal in degrees the quantity of a component a, then, m thecase of three Kelvin 2321;111:111} vrgeltillst r(11;:fgijrefidblysubscripts l, 2 and 3, the 40 K lztllilenmelting temperature of thecable shoe in degrees 7 a h q +a h2gz+a3h3g3 k=a function of the productof the specific heat and the a +a +a s ecific gravity of the soldermetal; and In the case of four different solders referred to as azthebase of natural logarithmic system' A, B, C and D the following tableindicates the composition of the solder, its melting point and the valueof M A Contact device according to claim 1 wherein as well as thepreferable value of k found. In the case has avalue of from to of solderB it was found that the same value of k as r A Contact device accordingto claim 1 wherein k for solder A is a good approximation. has a valueof 0.20 and the solder is silver solder.

Solder Cu, Zn, P Ag, 1 Cd, Mn, 00, Ni, Meltp.,C. M k

Percent Percent Percent Percent Percent Percent Percent As a suitableapproximation for the preferable value No references cited.

of k, it may be assumed that k increases linearly from 0.20 to 0.26 WhenM varies from 0.64 to 0.82.

For use with a silver solder metal having a melting ARTHUR L. LA POINT,Primary Examiner. R. A. BERTSCH, Assistant Examiner.

1. A CONTACT DEVICE FOR SOLDERING TO A METALLIC SURFACE COMPRISING ACONTACT SHOE HAVING AN APERTURE PROVIDED THEREIN INTO WHICH IS INSERTEDA METAL STUD HAVING A CYLINDER OF SOLDER METAL AT ITS FORWARD END, SAIDCONTACT SHOE BEING PROPORTIONED TO HAVE MINIMUM VOLUME ACCORDING TO THEFOLLOWING FORMULA: